1. Field of the Invention
The present invention relates to an automatic gain control circuit capable of receiving two broadcast signals of different systems as in a television signal of a digital system and a television signal of an analog system, for example.
2. Description of the Related Art
A prior art automatic gain control circuit will be explained in accordance with FIG. 2. A tuner 21 is a tuner capable of receiving both broadcast signals: a television signal (hereinafter called xe2x80x9canalog broadcast signalxe2x80x9d) of an analog system and a television signal (hereinafter called xe2x80x9cdigital broadcast signalxe2x80x9d) of a digital system. The tuner 21 has an intermediate frequency signal output terminal (hereinafter called xe2x80x9cIF signal output terminalxe2x80x9d) 21a and an automatic gain control voltage input terminal (hereinafter called xe2x80x9cAGC voltage input terminalxe2x80x9d) 21b and is electrically connected to an antenna 22. Further, the tuner 21 has a gain variable amplifier 21c, a mixer (not shown) for converting an input broadcast signal into an intermediate frequency signal (hereinafter called xe2x80x9cIF signalxe2x80x9d), etc. The gain variable amplifier 21c serves as an amplifier capable of varying amplification according to the magnitude of an automatic gain control voltage (hereinafter called xe2x80x9cAGC voltagexe2x80x9d) inputted from the AGC voltage input terminal 21b. When the AGC voltage is high, the amplifier serves so as to increase the amplification.
Further, the analog broadcast signal or digital broadcast signal inputted from the antenna 22 is amplified by the gain variable amplifier 21c according to the magnitude of the AGC voltage and converted into the intermediate frequency signal by the mixer, which in turn is outputted from the IF signal output terminal 21a. 
A high-frequency switch 23 is electrically connected to the IF signal output terminal 21a. The high-frequency switch 23 has output terminals 23a and 23b and an input terminal 23c and electrically connects the input terminal 23c to either the output terminal 23a or 23b according to the type of each received broadcast. Further, when the tuner 21 receives the analog broadcast signal therein, the high-frequency switch 23 serves so as to electrically connect the input terminal 23c to the output terminal 23a. When the tuner 21 receives the digital broadcast signal therein, the high-frequency switch 23 serves so as to electrically connect the input terminal 23c to the output terminal 23b. The high-frequency switch 23 can be constructed by means of a switch diode, for example.
A detector 24 for outputting an AGC voltage according to the level of the IF signal when the IF signal is inputted thereto, and a demodulator 25 for outputting a voice or speech signal and a video signal from the IF signal of the analog broadcast signal are electrically connected to the output terminal 23a. Thus, the IF signal is to be inputted to both the detector 24 and the demodulator 25.
Here, the detector 24 outputs the AGC voltage to be outputted therefrom as high as practicable when the input IF signal is low in level. On the other hand, when the input IF signal is high in level, the detector 24 outputs the AGC voltage to be outputted therefrom as low as practicable. Further, the AGC voltage outputted from the detector 24 is to be inputted to the AGC voltage input terminal 21b of the tuner 21 through an automatic gain control voltage selector switch (hereinafter called xe2x80x9cAGC voltage selector switchxe2x80x9d) 26.
Similarly, a detector 27 for outputting an AGC voltage when the IF signal is inputted thereto, and a demodulator 28 for outputting a voice or speech signal and a video signal from the IF signal of the digital broadcast signal are electrically connected to the output terminal 23b. Thus, the IF signal is to be inputted to both the detector 27 and the demodulator 28.
Here, the detector 27 outputs the AGC voltage to be outputted therefrom as high as practicable when the level of the input IF signal is low, whereas when the level of the input IF signal is high, the detector 27 outputs the AGC voltage to be outputted therefrom as low as practicable. Further, the AGC voltage outputted from the detector 27 is to be inputted to the AGC voltage input terminal 21b of the tuner 21 through the AGC voltage selector switch 26.
The AGC voltage selector switch 26 performs switching in interlock with the high-frequency switch 23 so that the AGC voltage outputted from the detector 24 or 27 is inputted to the gain variable amplifier 21c of the tuner 21. The AGC voltage selector switch 26 has input terminals 26a and 26b and an output terminal 26c and electrically connects the output terminal 26c to either the input terminal 26a or 26b according to each received broadcast. When the tuner 21 receives the analog broadcast signal therein, the AGC voltage selector switch 26 electrically connects the output terminal 26c to the input terminal 26a. When the tuner 21 receives the digital broadcast signal therein, the AGC voltage selector switch 26 electrically connects the output terminal 26c to the switching terminal 26b. Since it is difficult to construct the AGC voltage selector switch 26 by a switch diode, for example, a relay or the like is used for the AGC voltage selector switch 26.
Since the AGC voltage outputted from the detector 24 increases when the level of the analog broadcast signal is lowered upon receiving the analog broadcast signal, for example, the amplification of the gain variable amplifier 21c of the tuner 21 is made high and hence the level of the IF signal to be outputted is set so as to be kept constant, whereby suitable speech and video signals are obtained.
In the same manner as described above even when the digital broadcast signal is received, since the AGC voltage outputted from the detector 27 increases when the level of the digital broadcast signal is lowered, the amplification of the gain variable amplifier 21c of the tuner 21 is made high and hence the level of the IF signal to be outputted is set so as to be held constant, whereby suitable speech and video signals are obtained.
Since, however, the prior art is constructed so that both the analog broadcast signal and the digital broadcast signal can be received by one tuner, it is necessary to perform the switching between the high-frequency switch 23 and the AGC voltage selector switch 26 in interlock with each other according to each received signal.
Therefore, the high-frequency switch 23 can be easily constructed by means of the switch diode. Since, however, the AGC voltage selector switch 26 is switched over by using mechanical switch means such as the relay or the like, a problem arises in that, for example, a rise in the cost thereof occurs and its wide installation location is needed.
With the foregoing problems in view, it is accordingly an object of the present invention to allow switching between AGC voltages owing to a simpler configuration without using the mechanical switch means for the AGC voltage selector switch 26 and without any restriction on its installation location and at low cost.
According to one aspect of this invention, for achieving the above object, there is provided an automatic gain control circuit, comprising: a tuner capable of receiving broadcast signals of different systems and for amplifying an intermediate frequency signal according to an increase or decrease in automatic gain control voltage when the automatic gain control voltage is inputted thereto and outputting the same therefrom; a first detector for outputting a first control voltage increased as the level of the intermediate frequency signal increases, when the intermediate frequency signal is inputted thereto; a second detector for outputting a second control voltage maximal when the intermediate frequency signal is not inputted thereto and lowered as the level of the intermediate frequency signal increases when the intermediate frequency signal is inputted thereto; a selector switch for selecting either the first detector or the second detector as a destination to output the intermediate frequency signal according to the received broadcasts; and a voltage converting circuit for receiving the first control voltage and the second control voltage therein and thereby outputting the automatic gain control voltage therefrom, and wherein when the intermediate frequency signal is inputted to the second detector, the second control voltage is outputted to the tuner as the automatic gain control voltage, and when the intermediate frequency signal is inputted to the first detector, a voltage increased or decreased according to an increase or decrease in the voltage corresponding to the difference between the maximum second control voltage outputted from the second detector and the first control voltage is outputted to the tuner as the automatic gain control voltage.
Further, according to the automatic gain control circuit of the present invention, the voltage converting circuit comprises a first resistor, a second resistor and a transistor, and one end of the first resistor is electrically connected to the collector of the transistor and the emitter of the transistor is grounded via the second resistor, and the second control voltage is inputted to the other end of the first resistor and the first control voltage is inputted to the base of the transistor, whereby the automatic gain control voltage is outputted from the collector of the transistor.
A typical one of the present invention has been shown in brief. However, the various inventions of the present application and specific configurations of these inventions will be understood from the following description.